The invention relates generally to a magnetic resonance imaging (MRI) apparatus, and more particularly, to a thermal management system for cooling a heat generating component, such as a gradient coil assembly, a RF coil assembly, and the like, of the MRI apparatus.
Exemplary superconducting magnet systems operating in an AC environment include a transformer, a generator, a motor, superconducting magnet energy storage (SMES), and a magnetic resonance (MR) apparatus. Although a conventional MR magnet operates in a DC mode, some MR magnets may operate under an AC magnetic field from the gradient coils when the gradient leakage field to the magnet is high. Such an AC magnetic field generates AC losses in the magnet. An illustrative discussion of exemplary details of the MR apparatus is presented, for explanatory purposes.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, or “longitudinal magnetization”, MZ, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated and this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx, Gy, and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received nuclear magnetic resonance (NMR) signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
The thermal management of the gradient coils is one of the biggest technical barriers in the development of the MR apparatus. The demand for larger patient spaces and better image qualities result in higher current densities, which lead to higher volumetric heat generation rates. The heat generated in the coils, and in particular the gradient coils, needs to be removed from the MR apparatus for safe and reliable operation of the device, as well as the comfort and safety of the patient. Excessive heat may cause rising temperatures that may soften the epoxy insulation. When a threshold temperature is reached at any location, the epoxy resin tends to melt, causing the system to lose its structural durability. Any softening in the insulation may promote electrical discharge and may cause device failure. Therefore, effective thermal management is important to keep the temperatures below acceptable limits.
One method of thermal management of MR apparatus is to provide air cooling of the gradient coils. However, air cooling is not sufficient for very high heat loads, such as the type produced in the gradient coils of the MR apparatus.
Another method of thermal management of the MR apparatus is to provide a hermetically sealed liquid cooling system. In a typical liquid cooling arrangement, the liquid is passed through liquid channels or inside the conductor for direct cooling. Typically, the cooling circuits are of serpentine scheme with either copper tubes or lengthy plastic tubes. The liquid must enter and travel axially along the MR cylinder or along the hollow conductor for proper cooling. Although liquid cooling is a feasible option for high heat loads, liquid cooling requires a large pump and manifolds to distribute coolant over the channels evenly for the best performance. In addition, this method requires complicated manifold systems to distribute the coolant flow uniformly and require multiple inlet/exit connections. These connections must be made electrically insulating to prevent forming a closed conducting loop which creates imaging artifacts. Further, the logistics of liquid cooling such as the manifold design, number of flow circuits, inlet/outlet positions for the coolant, and the like, can interfere with the design space of other MR components and increase the overall complexity, cost and reliability.
It would therefore be desirable to provide a simpler and cost effective thermal management system to maintain, for example, gradient coil temperature within a specified range regardless of the selected excitation applied, thereby increasing system reliability, enabling higher power applications for faster imaging with improved image quality, longer scanning times, while providing for the comfort and safety of the patient.